1. Field of the Invention
The present invention relates to a lens barrel of the type comprising a lens barrel body including an axially movable lens unit, operational means for moving the lens unit and a handling load change mechanism provided between the operational means and the lens barrel body.
2. Description of the Prior Art
According to prior-art lens barrels, focusing is carried out when a moving lens unit is shifted along the optical axis. In the case of the prior-art lens barrel also adapted for a zoom lens, the focal length of the lens is varied as the moving lens unit is shifted along the optical axis.
With the above-described lens barrels, when carrying out high angle or low angle photography, e.g. close-up photography under a condition in which the optical axis is considerably angularly displaced from the horizontal, it is desirable that resistance to movement or the handling load of the moving lens unit be relatively heavy in order to prevent unintentional movement of the moving lens unit caused by the weight of the lens unit per se. On the other hand, under the normal photographic condition in which the optical axis is substantially horizontal, it is desirable that the handling load be relatively light in order to facilitate a quick operation. Therefore, various constructions have been provided for changing the handling load in accordance with photographic conditions.
The handling load change mechanisms of the prior-art lens barrels are constructed as follows. An elastic member supported by a member provided between an operational control member and a lens barrel body is elastically adjustable to vary the frictional pressure between the elastic member and the lens barrel body, whereby the handling load is changed in accordance with the varied frictional pressure (for example as disclosed in the Japanese utility model application laid open under No. 60-68509).
According to the above construction, however, there occur problems to be described hereinafter.
The handling load is changed by the adjustment of the elastic member by an amount which is extremely small. Further, the adjustment is carried out in the spare space between a member of the operational control member and a member of the lens barrel body so that an assembly error in these members considerably affects the amount of the handling load change. Therefore, it is difficult to set a predetermined handling load at the time of assembly and there often occur differences in the handling load among lens barrels. Also, the range of changeable handling load is rather limited.
Moreover, since the handling load is changed by the transformation of the elastic member, the elastic member may incur creep after a long use. Also, since the elastic member is in contact with a member of the lens barrel body and the handling load change operation is carried out against the frictional force caused by the contact pressure, abrasion of the contact members may unintentionally provide a handling load different from the initially set value.
Also, some of the aforementioned prior-art lens barrels comprise automatic operational means for permitting a driving device to move the movable lens unit and power transmission interruption means for interrupting a power transmission from the driving device, thereby to enable an autofocusing operation.
These types of lens barrels are adapted to be switched between an autofocusing mode where the driving device moves the movable lens unit and a manual focusing mode where the movable lens unit is moved by manually operating a focusing ring and the like, whereby, in the manual focusing mode, by breaking the operative connection between the driving device and the movable lens unit a handling load on manual lens moving operational means may be minimized thereby to provide a smooth manual focusing operation and at the same time the driving device is not forcibly operated.
This type of lens barrel is known in which a driven member for transmitting a driving force of the driving device to the movable lens unit is adapted to be switched along the direction of the drive axis between an operatively-connected state and a disconnected state with the driving device (e.g. a Japanese utility model application laid open under No. 58-180537).
On the other hand, in the case of the lens barrels capable of providing autofocusing, it is desirable in terms of durability and power consumption of the driving device that the operational load on the driving device which is actually a motor should be light. To this end, there is a known lens barrel in which the operational load on the automatic lens moving operational means in the course of the lens moving operation by the driving device is lightened by providing a frictionless member such as a ball on a sliding contact portion of the automatic lens moving operational means (e.g. a Japanese utility model application laid open under No. 57-187418).
However, there is a problem to adopt the above construction to a lens barrel that can be manually operated.
That is to say, since the manual lens moving operational means and the automatic lens operational means partially both utilize the portion directly acting on the movable lens unit, the light operational load in the autofocuing mode necessarily results in a light handling load in the manual focusing mode. Moreover, if the operative connection with the driving device is broken in the manual focusing mode, the handling load becomes even lighter, whereby the movable lens unit is considerably moved even by a small force, whereby accurate focusing is difficult or the movable lens unit is unintentionally moved out of the focused condition.